[switch] Improved design of switch

This fix #106

The new commands are:

$ opam switch -install {compiler}           //to install
$ opam switch -clone {compiler}             //to clone the packages from {compiler} to the current tree
$ opam switch -remove {compiler}            //to remove a compiler
$ opam switch -reinstall {compiler}         //to reinstall a compiler
$ opam switch {compiler}                    //to switch
$ opam switch -list                         //to list all available compilers
$ opam switch -current                      //to display the current compiler